1. Field of the Invention
This invention relates to a sensor system for monitoring fluid level and displacement and, more particularly, to a sensor system for monitoring the level of crude oil in storage containers and displacement of parts.
2. Background Art
There are many types of sensors known in the art for monitoring fluid level and displacement of parts, and especially for measuring the level of crude oil in storage containers. Many of these sensors utilize a float designed to interact with the sensor. These sensors can be expensive in order to obtain the accuracy necessary and are often affected by the fluids and other contaminates associated with storage containers in which such fluids are stored. The following is a listing of known sensors for monitoring or measuring fluid level and disadvantages of such sensors.
Linearly variable differential transformers (LVDTs) require high precision manufacturing of the coils and a sensor length of more than two times the useful length of the sensor. They also have resolution that is limited by the resolution of their data acquisition system and by the electrical noise of the whole system.
Ultrasonic transducers are affected by changes in pressure, temperature and other variations in the composition of the media in which they operate due to their sensitivity to the density of the media. This limitation thereby increases the probability of errors.
Reed switch arrays, used in the oil industry, provide an incremental readout with limited resolution. However, they are sensitive to shock and vibrations and can be damaged by electrical storms. Furthermore, they are labor intensive to manufacture, which makes them expensive, and are unreliable due to the hundreds of switch contacts and internal connections. The accuracy of such arrays is typically +/xe2x88x926.4 mm and clearance required between a float used with such reed switch arrays and the sensor elements must be between 0-3 mm. The arrays also require yearly cleaning and float replacement due to contaminant buildup.
Optical encoders are sensitive to contamination and are expensive. They also require high precision during manufacturing and implementation.
Magnetostrictive wave guide transducers are expensive and require high precision electronics. Also, the clearance between the float and the sensing element is limited.
Radar is expensive and has limited accuracy.
Capacitive probes are expensive and very sensitive to contamination. They also require high precision electronics and have a limited range.
Pressure transducers can be affected by contamination and have a resolution limited by the acquisition system employed.
It is an object of the present invention to provide a displacement measurement system that is inexpensive and easy to manufacture, has high reliability and accuracy, is easy to implement, and has a low sensitivity to contamination, shock, electrical storms and the media in which it operates.
It is an object of the present invention to provide a displacement measurement system having an element that includes a resonator circuit.
It is an object of the present invention to provide a displacement measurement system having two or more elements, with each element having a resonator circuit responsive to a different frequency excitation signal.
It is an object of the present invention to provide a displacement measurement system having two or more elements, with each element having a resonator circuit responsive to a common excitation frequency, with each resonator circuit activatable independent of the resonator circuits of other elements.
It is an object of the present invention to provide a method for measuring displacement that avoids measurement errors caused by the displacement measurement system of the present invention or by interaction thereof with the environment in which it is installed.
In accordance with one aspect of the invention, a sensor system for measuring displacement includes a primary coil wound around a longitudinally extending axis. At least one secondary coil is wound around the longitudinal axis. Each secondary coil has a winding density distribution that varies between the ends thereof. Each secondary coil has a winding density distribution that varies between a clockwise winding direction and a counterclockwise winding direction between the ends thereof. A coupler is positioned adjacent the primary coil between the ends thereof. The coupler includes a resonating circuit configured to resonate at a resonating frequency. At least one of the coupler and the coils are configured to move relative to the other of the coupler and the coils. A control system excites the primary coil with a first step of a signal and receives from each secondary coil in response thereto a time varying signal. Each time varying signal includes a ringing component superimposed on a time varying component temporally adjacent the first step of the signal. The control system acquires one or more values of each time varying signal after the ringing component thereof dissipates and determines therefrom a position of the coupler along the longitudinal axis.
The control system can also determines from the one or more values of each time varying signal a first peak value after the ringing component dissipates and a second peak value after the first peak value. The control system can determine for each time varying signal a position signal value from the first peak value and the second peak value. From the position signal value for the time varying signal, the control system determines a position of the coupler along the longitudinal axis.
In response to the first step of the signal, the primary coil produces a first step electromagnetic field. In response to receiving the first step electromagnetic field, the resonating circuit produces a time varying electromagnetic field at the resonating frequency. In response to receiving the first step electromagnetic field and the time varying electromagnetic field, each secondary coil produces its corresponding time varying signal.
After acquiring the one or more values of the time varying signal, the control system can excite the primary coil with a second step of the signal having a transition opposite the first step of the signal. After the second step of the signal, the control system can determines from the one or more values of each time varying signal another first peak value after another ringing component thereof dissipates and can acquire another second peak value after the first peak value. For each time varying signal, the control system can determine another position signal value from the other first peak value and the other second peak value. From the other position signal value for each time varying signal, the control system can determine a position of the coupler along the longitudinal axis.
The control system can also determine from the one or more values for all or part of at least one half cycle of each time varying signal an average value or an integral value. For each time varying signal, the controller can determine a position signal value from the average value or the integral value, and from the position signal value for each time varying signal can determine a position of the coupler along the longitudinal axis.
After acquiring the one or more values of each time varying signal, the controller can excite the primary coil with a second step of the signal having a transition opposite the first step of the signal. Thereafter, the control system can acquire one or more other values of each time varying signal after another ringing component thereof dissipates. From the other one or more values, the controller can determine for all or part of at least one half cycle of each time varying signal another average value or another integral value. For each time varying signal, the controller can determine another position signal value between the other average values or the other integral values and from the other difference value for each time varying signal can determine a position of the coupler along the longitudinal axis.
In accordance with another aspect of the invention, a method of detecting the position of a magnetically susceptible element is provided. In the method, a primary coil wound around a longitudinal axis and at least one secondary coil wound around the longitudinal axis at a variable winding density distribution are provided. A coupler is positioned along the longitudinal axis adjacent the primary coil and each secondary coil. The coupler includes a resonating circuit configured to resonate at a resonating frequency. The primary coil is excited with a first step of a signal and a time varying signal is received from each secondary coil. Each time varying signal has a ringing component superimposed on a time varying component temporally adjacent the first step of the signal. One or more values of each time varying signal are acquired after the ringing component thereof dissipates. A position of the coupler along the longitudinal axis is determined from the one or more values of each time varying signal.
Preferably, for each time varying signal, a position signal value is determined from a first peak value and a second peak value and the position of the coupler along the longitudinal axis is determined from the position signal values.
In accordance with another aspect of the invention, a displacement measuring sensor system is provided. The sensor system includes a first member extending along a longitudinal axis. A primary coil and a secondary coil are wound around the longitudinal axis of the first member between the ends thereof. A control system is connected for exciting the primary coil with an excitation signal and for receiving a time varying signal from the secondary coil. The time varying signal includes a ringing component superimposed on a time varying component temporally adjacent a transition of the excitation signal. A second member is positioned on or adjacent the longitudinal axis. The second member includes a resonator which causes the secondary coil to generate the time varying component in response to the excitation of the primary coil with the excitation signal. The control system delays sampling of the time varying signal until the ringing component thereof dissipates.
Preferably, the first element or the second element are configured to move relative to the other of the first element and the second element.